1. Field of the Invention
This invention relates to the treatment of oil wells, gas wells, injection wells and similar boreholes. In one aspect it relates to a method of stimulating the productivity of hydrocarbon-bearing formations by hydraulic fracturing techniques. In a more specific aspect, it relates to a method for terminating a fracturing-propping treatment when a selected portion of the injected treatment fluid reaches the perforations through which the treatment fluid enters the formation. Use of this method prevents overdisplacement and underdisplacement of propping agent particles in hydraulic fracturing treatment.
2. Description of the Prior Art
A common technique for stimulating the productivity or injectivity of subterranean formations traversed by a borehole involves a treatment known as hydraulic fracturing. In this treatment, a fluid is injected through the well and into the formation at a sufficiently great pressure and rate to cause the formation to fail in tension. This establishes cracks, commonly termed "fractures", in the formation. The earth stresses are normally such that the resulting fractures are vertical, extending in opposite directions from the well. The fracture can be extended several hundred feet into the formation, depending upon the volume and properties of the treating fluid and the nature of the formation. The fracture is normally propped open by means of particles known as propping agents. These particles are also referred to as "proppant". The propping agent is carried down the well and into the formation as a suspension in the fracturing fluid. The propping agent-treating fluid mixture is often referred to as a "proppant slurry". THe injection of proppant-bearing fluid is typically followed with the injection of proppant-free displacement fluid. As will be detailed below, the injection of this displacement fluid is preferably terminated as the last of the proppant reaches the perforations.
As the fracturing fluid bleeds off into the formation, the propping agent is deposited in the fracture. Upon the release of the fluid pressure, the fracture walls close upon the propping agent. The propping agent prevents the fracture from completely closing, thereby maintaining a channel in the formation which is conducive to fluid flow. If properly performed, the hydraulic fracturing treatment can increase the productivity of a well several fold.
A problem associated with the placement of the propping agent in a fracture is that of overdisplacement. As pointed out in SPE Paper 3030 "Stresses and Displacements Around Hydraulic Fractured Wells" published by the Society of Petroleum Engineers of the AIME in 1970, the tendency of a fracture to close in the near-wellbore region can greatly affect productivity. If the fracture is not completely filled with propping agent in the near wellbore region, the productivity of the well will be greatly reduced relative to what it would have been had an adequate amount of proppant remained in that portion of the fracture nearest the wellbore. This overdisplacement of proppant occurs when fluid not containing proppant is forced into the fractures. This proppant-free fluid tends to displace the proppant existing in the fracture to a position away from the wellbore. This leaves the near-wellbore region depleted in proppant. Studies have shown that the stress level in this region causes the fracture to close upon incomplete fracture fill-up, reducing the effectiveness of the treatment.
On the other hand, if too large a volume of propping agent is used, excess propping agent will settle in the wellbore. If the amount of excess propping agent is sufficiently great, it can occupy the wellbore to a level above the perforations, causing a reduction in well productivity. To remedy this situation, an expensive sand washing operation is generally required.
The normal technique for preventing overdisplacement of the slurry is to carefully monitor the volume of fluid pumped into the well so that upon injection of the proper volume of displacement fluid, the pumping operations are terminated. The proper displacement volume is based upon tubular volume calculations. However, the instruments, including flowmeters, tank strapping techniques, etc., used to measure the total volume of displacement fluid are not precise. Because of the inherent inaccuracies in these instruments, the monitoring technique often does not prevent underdisplacement or overdisplacement of proppant in the fractures.